ABSTRACT
Direct current (DC) has been used for high-voltage transmission circuits since 1954. Power transfer by high-voltage direct current (HVDC) is used to connect asynchronous AC systems and for circuits of underground or submarine high-voltage cables longer than about 50 km. DC transmission can be used to interface two AC networks or to bring power from a DC source such as a PV plant to load centres. Vector control schemes are usually employed for the inner current control. A conventional vector control scheme uses direct and quadrature (dq) components those are constant in the steady state rather than three-phase sinusoidal signals. A phase-locked loop (PLL) is used to lock the dq quantities onto the phases of the AC system. A DC microgrid can operate either autonomously or in grid-connected mode. Demand-supply matching is achieved easily in grid-connected mode. Energy storage is usually a combination of high power and high energy storage.
